1. Field of the Invention
This invention relates to pedagogic shop apparatus for instruction in plastics forming technology and more particularly to a blow molding apparatus suitable for such purposes.
2. Brief Description of the Prior Art
Previous attempts at teaching various techniques of the fabrication of plastic products to young pupils, e.g. high school and middle or junior high school students, resulted in the realization that in most instances the utilization of actual production equipment proved ineffectual.
The methods and apparatus of blow molding fabrication represent a substantial area of technology suitable for instruction to students. Modern blow molding methods are hybrids of plastics processing and glass blowing technology. Initial attempts at blow molding hollow objects such as bottles from a charge of molten plastic in a manner similar to that used in glass blowing proved ineffectual since the plastic material did not have a propensity for providing uniformity of wall thickness upon expansion as does glass.
Successful results were achieved through the utilization of a hollow closed end tube of viscous plastic known as a parison and blowing the viscous parison within a hollow mold. Parisons have been produced by heat sealing the bottom of an open viscous tube formed by continuous or ram extrusion. Alternatively, parisons were produced by injection molding a closed end tube.
Instruction in the principles and theory of blow molding could not readily be accomplished with student operation of actual production machinery. Such machinery was generally quite expensive, cumbersome, and much too intricate for the purpose of explanation and illustration of the various stages of the process. In some instances the complexity of such equipment was such that the desired pedagogic objectives were obfuscated. Further, since the majority of such machinery was automated to some extent, student control of stages of the molding cycle was not attainable.
Additionally, commercial blow molding equipment was inappropriate for school shop usage, since the primary design objective was production runs of commercial products utilizing custom designed molds. Set up of such equipment was quite complex and time consuming; however in commercial applications set up costs were amortized over the production run.
A prime factor in commercial blow molding cycles is the time required for cooling of parts. In many commercial applications, cooling time often constitutes as much as 90% of the total cycle time, hence cooling efficiency has become a significant factor in the design of molds and blow molding equipment. For this reason most blow molding equipment utilized water cooled molds having appropriate partitions or drilled channels to accommodate a flow of cooling water. Understandably, commercial blow molding equipment was unsuitable for use in classroom environments due to equipment costs and complexity of operation.
Students in a school shop did not require automated machinery. Nonautomated equipment capable of performing the actual processes, yet simple in design without complexities such as water cooling apparatus, was required. Such equipment would ideally allow students to operate the machinery in classroom surroundings permitting observation and participation in the various stages of a production cycle. Also of significant educational value is a pedagogic machine wherein the student actually attempts setting up the equipment for a particular project.
Other problems encountered with attempts to adapt prior machinery for pedagogic use have been the loss of the student's interest usually attributed to the performance of perfunctory tasks in a fabricating cycle of a machine set up by the instructor another student.
When students actually did set up molds in school shop molding equipment, it has been found that they were prone to error and various difficulties were encountered. Particularly with blow molding equipment, where a pair of mold segments were mounted for reciprocal movement to mate in registration over a hot extruded parison, proper realignment of mold segments after mold separation was difficult for students.